The present invention relates to sliding door frame structures, primarily sliding glass doors which are commonly used as patio doors and the like.
Sliding door assemblies normally have two or more panels, at least one of which slides, along a track, supported in a frame. To allow proper door functioning, the frame must be rigid and strong enough to withstand substantial perpendicular loading, due not only to environmental loading, i.e. wind loads, but also to forces exerted during use. The problem of bending of the frame members is complicated by their lengths, which in sliding doors can be 8-12 feet. As a result, loads applied to the frame can, depending upon how the frame is installed in the jamb, create substantial bending moments.
In addition to perpendicular loads, the frame must be capable of withstanding axial stresses imparted during opening and closing movement of the sliding door or doors, such that the track and frame remain rigid.
For aesthetic effect, the door panels, when used as outside doors, are formed almost entirely of glass, which results in panes of substantial size. Due to the large glass area, each of the panes is necessarily extremely heavy, on the order of 250 lbs. per pane. This requires a frame structure which is strong enough to support substantial weight, and durable to hold up to repetitive door opening and closing motions under substantial stress.
Concern for thermal insulation of doors and windows, through which considerable heat loss can occur, coupled with the fact that sliding glass doors have a large glass surface area and glass is a relatively poor insulator of heat, has almost necessitated the use of double or triple glazing in the door panels. This not only complicates the frame configuration, but the substantial additional weight resultant from the added panes of glass places significant additional strain on the frame support members. It is for this reason too, thermal efficiency, that it is necessary to retain rigidity of the frame and door members to resist bending under wind loads as would cause excess infiltration of air (and water), and loss of heat.
These heavy load and stress factors have severely inhibited the use of rigid plastic as the frame and sash members in the sliding glass door assembly, although from the standpoint of cost and ease of manufacture the use of a rigid plastic frame and sash would be greatly desirable. However, in order to be capable of withstanding the in-use conditions of the sliding glass door, the plastic members would have to have a relatively thick construction. Alternatively, if plastic members of relatively thin wall thickness were used, metal reinforcement members would be required to prevent bending or cracking of the plastic. In either case, the cost savings advantage of using plastic in the first place would be obviated due both to material costs and construction costs.